Now run certbot-auto, when you run first time, it will download some required packages and install it.

certbot-auto --help

I am going to install SSL for server with hostname server18.hostonnet.com, do generate SSL certificate, run

certbot-auto certonly -d server18.hostonnet.com

This will ask your email address. Since Proxmox VE server have no apache running, select option to start a temportary web server for SSL verification.

When SSL created, you see a confirmation like

IMPORTANT NOTES:
– Congratulations! Your certificate and chain have been saved at
/etc/letsencrypt/live/server18.hostonnet.com/fullchain.pem. Your
cert will expire on 2017-03-03. To obtain a new or tweaked version
of this certificate in the future, simply run certbot-auto again.
To non-interactively renew *all* of your certificates, run
“certbot-auto renew”
– If you lose your account credentials, you can recover through
e-mails sent to admin@hostonnet.com.
– Your account credentials have been saved in your Certbot
configuration directory at /etc/letsencrypt. You should make a
secure backup of this folder now. This configuration directory will
also contain certificates and private keys obtained by Certbot so
making regular backups of this folder is ideal.
– If you like Certbot, please consider supporting our work by: